The present invention relates to a fuel control system for a gas turbine, and more particularly, to a fuel control system suitable for a gas turbine for power generation.
When using a gas turbine as a power generator, a fuel control signal is determined in accordance with a difference between the actual rotational speed of its output turbine shaft and a predetermined set rotational speed thereof in order to maintain the frequency of output power of the turbine constant, thereby controlling the opening of a fuel control valve provided at a main fuel path leading to a fuel injection valve from a fuel tank in response to the fuel control signal so as to maintain the rotational speed of the output turbine constant.
However, there is a problem that when the load on the output turbine shaft drastically or abruptly increases as in the case of throwing a heavy load to a generator, the degree of opening of the fuel control valve can not be changed immediatly due to a mechanical delayed response even when the fuel control signal changes abruptly. As a result, the amount of fuel does not increase abruptly, thus lowering the frequency. In order to solve the above problem, according to the prior art, a system has been proposed in which a sub-fuel path which bypasses the fuel control valve is provided and an ON-OFF valve having a good response is provided in the sub-fuel path so as to rapidly increase the amount of fuel.
However, in this system there is a problem in that when the load abruptly increases, the total amount of the fuel passing through the fuel control valve and the one passing through the ON-OFF valve is applied to the fuel injection valve, while in this case because of the lowering of the number of rotation of the output turbine shaft during an abrupt increase of the load, the fuel control signal increases so that the opening of the fuel control valve becomes large.
FIGS. 1 (A) through (D) shows these conditions. Namely, when the load increases as shown in FIG. 1 (A), the amount of fuel passing through the fuel control valve varies as shown in FIG. 1 (B). In this case, the amount of fuel passing through the ON-OFF valve varies as shown in FIG. 1 (C). However, as shown in FIG. 1 (D), a peak of the fuel flow is produced just before the closing of the ON-OFF valve, and thus the fuel amount supplied to the injection valve causes the turbine to be entered into surge limit.
By contraries, when trying to suppress the peak so as not to enter the surge zone, the most effective amount of the fuel will be supressed just after or before the abrupt increase of the load, for the purpose of preventing the lowering of the number of rotation of the output turbine shaft because of the necessity of reducing the amount of the fuel passing through the ON-OFF valve.